Methods for manufacturing well structures for low-noise chemical sensors

ABSTRACT

In one implementation, a method for manufacturing a chemical detection device is described. The method includes forming a chemical sensor having a sensing surface. A dielectric material is deposited on the sensing surface. A first etch process is performed to partially etch the dielectric material to define an opening over the sensing surface and leave remaining dielectric material on the sensing surface. An etch protect material is formed on a sidewall of the opening. A second etch process is then performed to selectively etch the remaining dielectric material using the etch protect material as an etch mask, thereby exposing the sensing surface.

BACKGROUND

The present disclosure relates to sensors for chemical analysis, and to methods for manufacturing such sensors.

A variety of types of chemical sensors have been used in the detection of various chemical processes. One type is a chemically-sensitive field effect transistor (chemFET). A chemFET includes a source and a drain separated by a channel region, and a chemically sensitive area coupled to the channel region. The operation of the chemFET is based on the modulation of channel conductance, caused by changes in charge at the sensitive area due to a chemical reaction occurring nearby. The modulation of the channel conductance changes the threshold voltage of the chemFET, which can be measured to detect and/or determine characteristics of the chemical reaction. The threshold voltage may for example be measured by applying appropriate bias voltages to the source and drain, and measuring a resulting current flowing through the chemFET. As another example, the threshold voltage may be measured by driving a known current through the chemFET, and measuring a resulting voltage at the source or drain.

An ion-sensitive field effect transistor (ISFET) is a type of chemFET that includes an ion-sensitive layer at the sensitive area. The presence of ions in an analyte solution alters the surface potential at the interface between the ion-sensitive layer and the analyte solution, usually due to the dissociation of oxide groups by the ions in the analyte solution. The change in surface potential at the sensitive area of the ISFET affects the threshold voltage of the device, which can be measured to indicate the presence and/or concentration of ions within the solution.

Arrays of ISFETs may be used for monitoring chemical reactions, such as DNA sequencing reactions, based on the detection of ions present, generated, or used during the reactions. See, for example, U.S. Pat. No. 7,948,015 to Rothberg et al., which is incorporated by reference herein. More generally, large arrays of chemFETs or other types of chemical sensors may be employed to detect and measure static and/or dynamic amounts or concentrations of a variety of analytes (e.g. hydrogen ions, other ions, compounds, etc.) in a variety of processes. The processes may for example be biological or chemical reactions, cell or tissue cultures or monitoring, neural activity, nucleic acid sequencing, etc.

A specific issue that arises in the operation of chemical sensor arrays is the susceptibility of the sensor output signals to noise. Specifically, the noise affects the accuracy of the downstream signal processing used to determine the characteristics of the chemical and/or biological process being detected by the sensors.

It is therefore desirable to provide devices including low noise chemical sensors, and methods for manufacturing such devices.

SUMMARY

In one implementation, a method for manufacturing a chemical detection device is described. The method includes forming a chemical sensor having a sensing surface. A dielectric material is deposited on the sensing surface. A first etch process is performed to partially etch the dielectric material to define an opening over the sensing surface and leave remaining dielectric material on the sensing surface. An etch protect material is formed on a sidewall of the opening. A second etch process is then performed to selectively etch the remaining dielectric material using the etch protect material as an etch mask, thereby exposing the sensing surface.

In another implementation, a chemical detection device is described. The device includes a chemical sensor having a sensing surface. A dielectric material has an opening extending to the sensing surface. A sidewall spacer is on a sidewall of the opening. The sidewall spacer has a bottom surface spaced away from the sensing surface and an inside surface defining a reaction region for receiving at least one reactant.

Particular aspects of one more implementations of the subject matter described in this specification are set forth in the drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of components of a system for nucleic acid sequencing according to an exemplary embodiment.

FIG. 2 illustrates a cross-sectional view of a portion of the integrated circuit device and flow cell according to an exemplary embodiment.

FIGS. 3A and 3B illustrate cross-sectional and top views respectively of a representative chemical sensor and corresponding reaction region according to an exemplary embodiment.

FIGS. 4 to 8 illustrate stages in a manufacturing process for forming a low noise chemical sensor and corresponding well structure according to an exemplary embodiment.

DETAILED DESCRIPTION

A chemical detection device is described that includes low noise chemical sensors, such as chemically-sensitive field effect transistors (chemFETs), for detecting chemical reactions within overlying, operationally associated reaction regions.

Applicants have found that a significant amount of the total noise in chemical sensors, such as chemFETs, can be attributed to etching processes involved in forming the overlying reaction regions. In particular, subjecting the sensing surface of a chemical sensor to prolonged periods of a high-energy directional etching process can cause significant noise in the sensor. For example, plasma impinging on the sensing surface can cause charge build up, to the point of causing undesirable changes or damage within the sensor. This accumulated charge can become trapped in the gate oxide and/or the gate oxide-semiconductor substrate interface of the chemFETs, thereby contributing to the noise and resulting in variations in operation and degradation in performance.

Techniques described herein can reduce or eliminate charge accumulation in the chemical sensors during the formation of the overlying reaction regions. In doing so, low noise chemical sensors with uniform performance across an array are provided, such that the characteristics of subsequent chemical reactions can be accurately detected.

FIG. 1 illustrates a block diagram of components of a system for nucleic acid sequencing according to an exemplary embodiment. The components include a flow cell 101 on an integrated circuit device 100, a reference electrode 108, a plurality of reagents 114 for sequencing, a valve block 116, a wash solution 110, a valve 112, a fluidics controller 118, lines 120/122/126, passages 104/109/111, a waste container 106, an array controller 124, and a user interface 128. The integrated circuit device 100 includes a microwell array 107 overlying a sensor array that includes chemical sensors as described herein. The flow cell 101 includes an inlet 102, an outlet 103, and a flow chamber 105 defining a flow path of reagents over the microwell array 107.

The reference electrode 108 may be of any suitable type or shape, including a concentric cylinder with a fluid passage or a wire inserted into a lumen of passage 111. The reagents 114 may be driven through the fluid pathways, valves, and flow cell 101 by pumps, gas pressure, or other suitable methods, and may be discarded into the waste container 106 after exiting the outlet 103 of the flow cell 101. The fluidics controller 118 may control driving forces for the reagents 114 and the operation of valve 112 and valve block 116 with suitable software.

The microwell array 107 includes an array of reaction regions as described herein, also referred to herein as microwells, which are operationally associated with corresponding chemical sensors in the sensor array. For example, each reaction region may be coupled to a chemical sensor suitable for detecting an analyte or reaction property of interest within that reaction region. The microwell array 107 may be integrated in the integrated circuit device 100, so that the microwell array 107 and the sensor array are part of a single device or chip.

The flow cell 101 may have a variety of configurations for controlling the path and flow rate of reagents 114 over the microwell array 107. The array controller 124 provides bias voltages and timing and control signals to the integrated circuit device 100 for reading the chemical sensors of the sensor array. The array controller 124 also provides a reference bias voltage to the reference electrode 108 to bias the reagents 114 flowing over the microwell array 107.

During an experiment, the array controller 124 collects and processes output signals from the chemical sensors of the sensor array through output ports on the integrated circuit device 100 via bus 127. The array controller 124 may be a computer or other computing means. The array controller 124 may include memory for storage of data and software applications, a processor for accessing data and executing applications, and components that facilitate communication with the various components of the system in FIG. 1.

The values of the output signals of the chemical sensors indicate physical and/or chemical parameters of one or more reactions taking place in the corresponding reaction regions in the microwell array 107. For example, in an exemplary embodiment, the values of the output signals may be processed using the techniques disclosed in Rearick et al., U.S. patent application Ser. No. 13/339,846, filed Dec. 29, 2011, based on U.S. Prov. Pat. Appl. Nos. 61/428,743, filed Dec. 30, 2010, and 61/429,328, filed Jan. 3, 2011, and in Hubbell, U.S. patent application Ser. No. 13/339,753, filed Dec. 29, 2011, based on U.S. Prov. Pat. Appl. No. 61/428,097, filed Dec. 29, 2010, which are all incorporated by reference herein in their entirety.

The user interface 128 may display information about the flow cell 101 and the output signals received from chemical sensors in sensor array on the integrated circuit device 100. The user interface 128 may also display instrument settings and controls, and allow a user to enter or set instrument settings and controls.

In an exemplary embodiment, during the experiment the fluidics controller 118 may control delivery of the individual reagents 114 to the flow cell 101 and integrated circuit device 100 in a predetermined sequence, for predetermined durations, at predetermined flow rates. The array controller 124 can then collect and analyze the output signals of the chemical sensors due to chemical reactions occurring in response to the delivery of the reagents 114.

During the experiment, the system may also monitor and control the temperature of the integrated circuit device 100, so that reactions take place and measurements are made at a known predetermined temperature.

The system may be configured to let a single fluid or reagent contact the reference electrode 108 throughout an entire multi-step reaction during operation. The valve 112 may be shut to prevent any wash solution 110 from flowing into passage 109 as the reagents 114 are flowing. Although the flow of wash solution may be stopped, there may still be uninterrupted fluid and electrical communication between the reference electrode 108, passage 109, and the microwell array 107. The distance between the reference electrode 108 and the junction between passages 109 and 111 may be selected so that little or no amount of the reagents flowing in passage 109 and possibly diffusing into passage 111 reach the reference electrode 108. In an exemplary embodiment, the wash solution 110 may be selected as being in continuous contact with the reference electrode 108, which may be especially useful for multi-step reactions using frequent wash steps.

FIG. 2 illustrates cross-sectional and expanded views of a portion of the integrated circuit device 100 and flow cell 101. During operation, the flow chamber 105 of the flow cell 101 confines a reagent flow 208 of delivered reagents across open ends of the reaction regions in the microwell array 107. The volume, shape, aspect ratio (such as base width-to-well depth ratio), and other dimensional characteristics of the reaction regions may be selected based on the nature of the reaction taking place, as well as the reagents, byproducts, or labeling techniques (if any) that are employed.

The chemical sensors of the sensor array 205 are responsive to (and generate output signals) chemical reactions within associated reaction regions in the microwell array 107 to detect an analyte or reaction property of interest. The chemical sensors of the sensor array 205 may for example be chemically sensitive field-effect transistors (chemFETs), such as ion-sensitive field effect transistors (ISFETs). Examples of chemical sensors that may be used in embodiments are described in U.S. Patent Application Publication No. 2010/0300559, No. 2010/0197507, No. 2010/0301398, No. 2010/0300895, No. 2010/0137143, and No. 2009/0026082, and U.S. Pat. No. 7,575,865, each which are incorporated by reference herein.

FIGS. 3A and 3B illustrate cross-sectional and top views respectively of a representative chemical sensor 350 in the sensor array coupled to a corresponding reaction region 301 in the microwell array according to an exemplary embodiment.

In the illustrated example, the chemical sensor 350 is an ion-sensitive field effect transistor. The chemical sensor 350 includes a floating gate structure 318 having a sensor plate 320 separated from the reaction region 301 by an ion-sensitive layer 316. In the illustrated example, the floating gate structure 318 includes multiple patterned layers of conductive material within layers of dielectric material 319. As described in more detail below, the upper surface of the ion-sensitive layer 316 acts as the sensing surface 317 for the chemical sensor 350.

The ion-sensitive layer 316 may be deposited using various techniques, or naturally grown during one or more of the manufacturing processes used to form the chemical sensor 350. In some embodiments, the ion-sensitive layer 316 is a metal oxide, such as an oxide of silicon, tantalum, aluminum, lanthanum, titanium, zirconium, hafnium, tungsten, palladium, iridium, etc.

The ion-sensitive layer 316 may for example be an oxide of the upper layer of conductive material of the sensor plate 220. For example, the upper layer of the sensor plate 320 may be titanium nitride, and the ion-sensitive layer 316 may comprise titanium oxide or titanium oxynitride. More generally, the ion-sensitive layer 316 may comprise a variety of different materials to facilitate sensitivity to particular ions. For example, silicon nitride or silicon oxynitride, as well as metal oxides such as silicon oxide, aluminum or tantalum oxides, generally provide sensitivity to hydrogen ions, whereas layers comprising polyvinyl chloride containing valinomycin provide sensitivity to potassium ions. Materials sensitive to other ions such as sodium, silver, iron, bromine, iodine, calcium, and nitrate may also be used.

The chemical sensor 350 also includes a source region 321 and a drain region 322 within a semiconductor substrate 354. The source region 321 and the drain region 322 comprise doped semiconductor material have a conductivity type different from the conductivity type of the substrate 354.

Channel region 323 separates the source region 321 and the drain region 322. The floating gate structure 318 overlies the channel region 323, and is separated from the substrate 354 by a gate dielectric 352. The gate dielectric 352 may be for example silicon dioxide. Alternatively, other dielectrics may be used for the gate dielectric 352.

An opening extends through dielectric material 310 to the ion-sensitive layer 316. The dielectric material 310 may comprise one or more layers of material deposited sequentially. The opening includes a lower portion 314 proximate to the ion-sensitive layer 316. An upper portion 315 of the opening extends from the lower portion 314 to the upper surface of the dielectric material 310.

The upper portion 315 of the opening includes a sidewall spacer 302 on a sidewall 303 of the dielectric material 310. As shown in FIG. 3A, the sidewall spacer 302 does not extend down the entire opening to the ion-sensitive layer 316. Instead, the sidewall spacer 302 has a bottom surface spaced away from the ion-sensitive layer 316 by the lower portion 314 of the opening.

The sidewall spacer 302 includes an inner surface 304 defining an upper segment of the reaction region 301. A lower segment of the reaction region 301 is defined by the lower portion 314 of the opening. As a result of this structure, the sidewall spacer 302 overhangs the lower portion 314 of the opening, such that the width of the lower segment of the reaction region 301 is greater than the width of the upper segment of the reaction region 301.

The opening through the dielectric material 310 is formed using a two step etching process, as described in more detail below with respect to FIGS. 4-8. A first etch process partially etches the dielectric material 310 to define the upper portion 315 of the opening, and leave remaining dielectric material 310 over the ion-sensitive layer 316. The first etch process may be a process which, if continued all the way down to the ion-sensitive layer 316, would cause significant charge to accumulate on the floating gate structure 318. However, by leaving remaining dielectric material 310 on the ion-sensitive layer 316, such charge accumulation is precluded. For example, first etch process may for example be a directional etching process such as RIE, so that the opening can have a high aspect ratio and be accurately and repeatedly defined, while also not contributing to noise in the chemical sensor 350.

The sidewall spacer 302 is then formed on the sidewall 303. The dielectric material 310 at the lower portion 314 of the opening comprises material that can be selectively etched relative to the material of the sidewall spacer 302. For example, in one embodiment, the dielectric material 310 at the lower portion 314 of the opening comprises silicon dioxide, and the sidewall spacer 302 comprises silicon nitride. Alternatively, other dielectric and/or electrically conductive materials may be used. For example, in some embodiments the sidewall spacer 302 may comprise an electrically conductive material such as titanium nitride. An electrically conductive material for the sidewall spacer 302 can reduce the thermal resistance of the reaction region 301 when containing solution, which in turn can reduce the overall thermal noise during operation.

The sidewall spacer 302 then serves as an etch protect layer to retain the shape of the upper portion 315 during a second etch process used to form the lower portion 314. This second etch process continues the opening and exposes the ion-sensitive layer 316 to define the reaction region 301.

The second etching process may for example be a wet etch process which does not contribute charge accumulation on the floating gate structure 318. Applicants have found that a significant amount of the total noise in ISFETs, can be attributed to the use of high-power directional etching processes involved in forming the reaction regions. In particular, using plasma to etch all the way down to the ion-sensitive layer 316 can subject the floating gate structure 318 to the plasma for prolonged periods of time. The plasma can cause charge build up on the floating gate structure 318, to the point of causing undesirable changes or damage to the device. This accumulated charge can become trapped in the gate dielectric 352 and/or the interface between the gate dielectric 352 and the semiconductor substrate 354, thereby contributing to the noise and resulting in variations in operation and degradation in performance.

By using a second etching process to which does not accumulate charge on the floating gate structure 318, noise induced in the chemical sensor 350 due to the formation of the reaction region 301 can be eliminated. As a result, the techniques described herein can be used to form low noise chemical sensors with uniform performance across an array, such that the characteristics of chemical reactions can be accurately measured.

As shown in the top view of FIG. 3B, in the illustrated example the opening and the reaction region 301 have circular cross sections. Alternatively, these may be non-circular. For example, the cross-section may be hexagonal.

In operation, the chemical sensor 350 is responsive to (and generates an output signal related to) the amount of a charge 324 present on ion-sensitive layer 316 opposite the sensor plate 320. Changes in the charge 324 cause changes in the voltage on the floating gate structure 318, which in turn changes in the threshold voltage of the transistor. This change in threshold voltage can be measured by measuring the current in the channel region 323 between the source region 321 and a drain region 322. As a result, the chemical sensor 350 can be used directly to provide a current-based output signal on an array line connected to the source region 321 or drain region 322, or indirectly with additional circuitry to provide a voltage-based output signal. Reactants, wash solutions, and other reagents may move in and out of the reaction region 301 by a diffusion mechanism 340.

In an embodiment, reactions carried out in the reaction region 301 can be analytical reactions to identify or determine characteristics or properties of an analyte of interest. Such reactions can generate directly or indirectly byproducts that affect the amount of charge adjacent to the sensor plate 320. If such byproducts are produced in small amounts or rapidly decay or react with other constituents, multiple copies of the same analyte may be analyzed in the reaction region 301 at the same time in order to increase the output signal generated. In an embodiment, multiple copies of an analyte may be attached to a solid phase support 312, either before or after deposition into the reaction region 301. The solid phase support 312 may be microparticles, nanoparticles, beads, solid or porous comprising gels, or the like. For simplicity and ease of explanation, solid phase support 312 is also referred herein as a particle. For a nucleic acid analyte, multiple, connected copies may be made by rolling circle amplification (RCA), exponential RCA, or like techniques, to produce an amplicon without the need of a solid support.

In various exemplary embodiments, the methods, systems, and computer readable media described herein may advantageously be used to process and/or analyze data and signals obtained from electronic or charged-based nucleic acid sequencing. In electronic or charged-based sequencing (such as, pH-based sequencing), a nucleotide incorporation event may be determined by detecting ions (e.g., hydrogen ions) that are generated as natural by-products of polymerase-catalyzed nucleotide extension reactions. This may be used to sequence a sample or template nucleic acid, which may be a fragment of a nucleic acid sequence of interest, for example, and which may be directly or indirectly attached as a clonal population to a solid support, such as a particle, microparticle, bead, etc. The sample or template nucleic acid may be operably associated to a primer and polymerase and may be subjected to repeated cycles or “flows” of deoxynucleoside triphosphate (“dNTP”) addition (which may be referred to herein as “nucleotide flows” from which nucleotide incorporations may result) and washing. The primer may be annealed to the sample or template so that the primer's 3′ end can be extended by a polymerase whenever dNTPs complementary to the next base in the template are added. Then, based on the known sequence of nucleotide flows and on measured output signals of the chemical sensors indicative of ion concentration during each nucleotide flow, the identity of the type, sequence and number of nucleotide(s) associated with a sample nucleic acid present in a reaction region coupled to a sensor can be determined.

FIGS. 4 to 8 illustrate stages in a manufacturing process for forming a low noise chemical sensor and corresponding well structure according to an exemplary embodiment.

FIG. 4 illustrates a first stage of forming a dielectric material 310 on the sensing surface 317 of a chemical sensor 350. In the illustrated example, the chemical sensor 350 is the ion-sensitive field effect transistor described above and includes ion-sensitive layer 316 having a top surface acting as the sensing surface 317. The chemical sensor 350 may for example be formed using the techniques described in U.S. Patent Application Publication No. 2010/0300559, No. 2010/0197507, No. 2010/0301398, No. 2010/0300895, No. 2010/0137143, and No. 2009/0026082, and U.S. Pat. No. 7,575,865, each which are incorporated by reference herein.

In the illustrated embodiment, the dielectric material 310 is formed by sequentially depositing a first layer 400 of silicon dioxide, a second layer 410 of silicon nitride, a third layer 420 of silicon dioxide, and a fourth layer 430 of silicon nitride. More generally, the dielectric material 310 may comprise one or more layers, and may comprise various materials.

Next, the dielectric material 310 of the structure in FIG. 4 is partially etched using an etch process to define an opening 500 over the sensing surface 317, resulting in the structure illustrated in FIG. 5. As shown in FIG. 5, the opening 500 includes a sidewall 303, and a bottom surface 502 spaced away from the sensing surface 317 by remaining dielectric material 310. This remaining dielectric material 310 prevents the sensing surface 317 from being subjected to this etch process, so that charge does not accumulate. As a result, the etch process used to form the opening 500 may be a directional etch process, such as a plasma etch, so that the opening 500 can be well defined, while at the same time precluding this etching process from increasing the noise in the chemical sensor 350.

The opening 500 may for example be formed by using a lithographic process to pattern a layer of photoresist on the dielectric material 310 to define the location of the opening 500, and then anisotropically etching the dielectric material 310 using the patterned photoreist as an etch mask. The anisotropic etching of the dielectric material 310 may for example be a dry etch process, such as a fluorine based Reactive Ion Etching (RIE) process.

In the illustrated example, the etching of the dielectric material 310 is carried out using RIE with end point detection, so that the etching can stop at or in the first layer 400. The etching may for example be performed using a single etch chemistry to each all the layers 400, 410, 420 and 430. Alternatively, different etch chemistries may be used for each of the layers.

Next, a conformal layer 600 of etch protect material is formed on the structure illustrated in FIG. 5, resulting in the structure illustrated in FIG. 6. In the illustrate embodiment, the conformal layer 600 comprises silicon nitride and is formed using plasma enhanced chemical vapor deposition (PECVD). Alternatively, other procedures and materials may be used. For example, the conformal layer 600 could be deposited by sputtering, atomic layer deposition (ALD), low pressure chemical vapor deposition (LPCVD), etc. As described in more detail below with respect to FIG. 8, the remaining dielectric material 310 over the sensing surface 317 (material of layer 400 in this example) comprises material which can be selectively etched relative to the material of the conformal layer 600 when subjected to a chosen etch process.

Next, an anisotropic etching process is performed on the conformal layer 600 illustrated in FIG. 6 to form a sidewall spacer 302 of remaining material of layer 600 within the opening, resulting in the structure illustrated in FIG. 7. The anisotropic etching process removes the material of the conformal layer 600 on horizontal surfaces at a faster rate than material on vertical surfaces. The anisotropic etching process may for example be performed using a RIE or other plasma etching process. Alternatively, other etching processes may be used.

Next, an isotropic etching process is performed on the structure illustrated in FIG. 7 to extend the opening 500 down to the sensing surface 317 using the sidewall spacer 302 as an etch mask, thereby forming the reaction region 301 and resulting in the structure illustrated in FIG. 8. The isotropic etching process selectively etches material of the layer 400, relative to the material of the sidewall spacer 302. As a result, the sidewall spacer 302 acts to protect and retain the shape of the upper portion of the opening.

The isotropic etching process may for example be a wet etch process, such as a buffered oxide etch, HF etch chemistry, etc. Alternatively, other etch processes and chemistries may be used.

A wet process results in no charge accumulation on the floating gate structure 318. As a result, noise induced in the chemical sensor 350 can be significantly reduced, as compared to the use of directional etch processes (e.g. plasma etching) to etch down to the sensing surface 317. In doing so, the techniques described herein can be used to form low noise chemical sensors with uniform performance across an array, such that the characteristics of subsequent chemical reactions can be accurately measured.

While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims. 

What is claimed is:
 1. A chemical detection device, comprising: a plurality of chemical sensors each having respective sensing surfaces; a dielectric material having openings extending to the sensing surfaces, the openings having sidewalls, the dielectric material having a top surface extending between adjacent openings; and a plurality of sidewall spacers on the sidewalls of the openings and not on the top surface of the dielectric material, the sidewall spacers having a bottom most surface spaced away from the sensing surfaces such that the sidewall spacer does not contact the surfaces, the sidewall spacers each having an inside surface defining a reaction region for receiving at least one reactant.
 2. The chemical detection device of claim 1, wherein the dielectric material within a lower portion of the openings comprises material different than that of the plurality of sidewall spacers.
 3. The chemical detection device of claim 1, wherein the chemical sensors are chemically-sensitive field effect transistors, and the sensing surfaces are coupled to respective channels of the chemically-sensitive field effect transistors.
 4. The chemical detection device of claim 3, wherein the sensing surfaces are on an upper surface of respective floating gates of the chemically-sensitive field effect transistors.
 5. The chemical detection device of claim 1, wherein the chemical sensors generate respective sensor signals in response to a chemical reaction occurring within a given reaction region.
 6. The chemical detection device of claim 5, wherein the chemical reaction is a sequencing reaction. 